V24A-01 INVITED
Plate tectonic constraints on the cessation of subduction beneath the Baja California peninsula, Mexico
I review published models, existing global plate tectonic data and published marine geophysical observations west of Baja California to assess the timing and conditions under which subduction ceased along the W margin of Baja California. The relative motion of the Farallon microplate fragments can be reconstructed using Pacific- North America global plate motions (from the Pacific-Antarctica-Nubia-North America plate circuit) added to the local velocities of the microplates with respect to the Pacific plate. Because the Pacific plate was moving obliquely away from North America, the time at which subduction stopped has often been taken to be the time at which the microplates joined the Pacific plate (the ages of dead spreading centers preserved west of North America on the Pacific plate). The timing of cessation of subduction west of what is now northern Baja California is not recorded by a dead ridge offshore but is inferred to be coincident with extension and rotation in the continental borderland (early-middle Miocene). The Arguello microplate stopped spreading relative to the Pacific plate at about 13 Ma, providing a younger age limit on the cessation of subduction in the sector N of the Shirley transform fault. The time of cessation of spreading of the Magdalena-Pacific (M-P) ridge has been proposed by Michaud et al. (2006 Geology) to be as young as 8 Ma. However, the clockwise rotation of the M-P ridge before it ceased, and its inferred slow spreading rate away from the Pacific plate implies transcurrent motion with virtually no convergence between the Magdalena microplate and the North America plate during the last stages of activity of the M-P ridge. Subduction can occur by motion of forearc fragments without any convergence of the major bounding plates (e.g., the modern South Shetland Trench), but this may be ruled out for Baja California due to the small spatial scale of the microplates compared to the scale of the stable Baja California peninsula block. Due to the progressively slower convergence rates in this region since 14 Ma, the formation of asthenospheric windows during waning subduction is likely to have been extremely important in the change from subduction-related to "post-subduction" magmatism and in its variability along strike in Baja California.
V24A-02
Proto-Gulf volcanic sequences from Sierra Suvuk and Cerro Ladrilleros. El Pinacate Volcanic Field, NW Mexico.
The Pinacate Volcanic Field (PVF) lies at the northern end of the Gulf of California and is characterized by a complex tectonic history reflected in two main volcanic episodes: (1)alkaline Pinacate event; (2) Lower to Middle Miocene sequences that comprise mesa basalts with transitional alkali character, calc-alkaline andesites- dacites, and high silica rhyolites evolving toward peralkaline liquids, called pre-Pinacate. Our purpose here is to describe the stratigraphy and structural control of the pre-Pinacate Miocene volcanic sequences at Sierra Suvuk (SS) and Cerro Ladrilleros (CL). Different volcanic pulses with calc-alkaline affinities have been recognized within the SS and CL outcrops: [A] a dome-related Suvuk volcanism represented by complete lithofacies variations, from central-internal to external facies with associated pyroclastic flows. This dacitic dome is composed by a predominant mineralogy of Pl > Px >> Anf. [B] A dome-related Ladrilleros volcanism that differs from the Suvuk lavas by the fact that Anf >> Px and by its transition toward rhyodacitic compositions to the East. [C] A Fluidal Ladrilleros volcanism represented by lava flows with Pl + Anf as a characteristic mineralogy, representing the final phase of activity at Cerro Ladrilleros (ca 12Ma). [D] A Basic Suvuk volcanism represented by basaltic sub-horizontal lava flows cropping out at the summit of the SS. These lavas enclose Pl + Ol and have a thickness of about 50 m. Finally, [E] a Basic Suvuk valley volcanism represented by sub-horizontal basaltic lava flows and dikes located at the level of the actual valley, on the southeast flank of SS. The calc-alkaline affinity of the SS and CL lavas is not directly related to subduction. The occurrence of such a magmatism in an extensional tectonic setting could be explained by the persistency of a metasomatized mantle source, result of prolonged subduction processes since the early cretaceous. The behaviors of post-subduction volcanism in the Gulf of California area, and the peculiar petrology of Sierra Suvuk and Cerro Ladrilleros lavas, help us to constrain the tectonic evolution of the region since the Middle Miocene.
V24A-03
Late Neogene Volcanic Stratigraphy in the Southern Puertecitos Volcanic Province of Baja California: Time Constraints and Vent Source Location
Late Neogene syn-rift explosive volcanism occurred in the Puertecitos Volcanic Province along the western margin of the Gulf of California. This volcanic episode is possibly related to extension during opening of the lower Delfin basin in mid-late Pliocene time. The volcanic stratigraphy in the southern Puertecitos Volcanic Province comprises three main groups: group 1 is a mid-Miocene, arc-related volcanic and sedimentary apron. Groups 2 and 3 are syn-rift volcanic units interstratified within alluvial conglomerate. Group 2 includes a non-welded, crystal-rich pyroclastic flow deposit, and a dark glassy dacite lava flow. Distinctive mineralogy of the crystal tuff is augite, sanidine-microcline and quartz. Two 39Ar/40Ar laser step-heating experiments on sanidine grains yielded an 6.18 ± 0.03 Ma isochron age, consistent with a 6.1 ± 0.4 Ma plateau age obtained in the dacite lava. Thickness of the crystal tuff varies from 35 m in the northeast to 10 m in the southwest along 5 km of distance. Group 3 is characterized by the lack of quartz and potassic feldspar phenocrysts. Three laser step heating experiments on groundmass samples constrain this pulse of explosive volcanism between 2.9 ± 0.1 and 2.3 ± 0.03 Ma. Thicknesses of individual units increase to the northeast and collectively reach up to 150 m. Isopath maps for distinctive flow-units indicate consistent dispersion direction to the SW (average azimuth 210° ± 15°). This inferred flow direction is similar to the orientation of magnetic susceptibility axes measured in 20 oriented samples that yield a mean azimuth of 214°± 24°. In group 3 flow-units eutaxitic foliation is concordant and dips 8-20° to the ENE. Tilting of the volcanic sequence is produced by a series of NNW-trending, west-dipping, high-angle normal faults with less than 40 m of throw. Balanced cross- sections in the southern Puertecitos Volcanic Province indicates that post-2.8 Ma extension is less than 15% suggesting that major deformation during the opening of the Lower Delfin basin has been accommodated to the east. Our data support multiple source vents located offshore the central Puertecitos Volcanic Province. These pyroclastic flows may constitute useful marker horizons in marine seismic lines for reconstructing the timing and amount of extension across conjugate margins in the Lower Delfin basin.
V24A-04 INVITED
From Subduction to Continental Breakup: Neogene and Quaternary Magmatic Evolution of Baja California Peninsula and the Adjacent Gulf of California, Mexico
After the subduction stopped at ca. 12.5 Ma off Baja California Peninsula, calc-alkaline volcanic activity vanished
progressively, and the Comondú volcanic arc was replaced by a post-subduction diversified volcanism which
derived from at least five distinct magmatic sources located beneath the Baja California Peninsula and the Gulf of
California. New field data, 40K-40Ar datings and geochemical and petrological studies of some volcanic fields
and isolated volcanic centers, including San Esteban island, allow us to define the following volcanic suites from
Late Miocene to Quaternary: 1) magnesian andesites, or "bajaites", 2) adakites, 3) tholeiitic basalts, 4) niobium-
rich basalts (NEB), 5) calc-alkaline andesites, and 6) alkali basalts, in order of decreasing volumetric abundance.
In the northern part of the Peninsula until the latitude of 29°N, where the oceanic ridge subduction has
been documented, volcanism is mainly represented by magnesian andesites and alkali basalts. To the south,
alkali basalts have not been reported, but, from Vizcaino peninsula to La Purísima volcanic field, we observe a
close association of Late Miocene magnesian andesites, adakites, niobium-rich basalts and tholeiitic basalts.
These rocks together indicate the presence of an asthenospheric window, related to the slab tear which
developed within the downgoing oceanic plate. Tholeiites are considered as a product of partial melting of the
lower plate asthenosphere, whereas adakites likely derive from partial melting of the edges of the slab window,
and NEB and magnesian andesites from the melting of the supraslab mantle following its interaction with
adakitic liquids. We propose that, after the oceanic accretion ceased off southern Baja California between 12 and
8 Ma, the slab tearing was due to the combination of the capture of the unsubducted remnant of the Magdalena
plate by the Pacific plate and of the progressive sinking of the deep subducted oceanic plate. The slab tearing
was probably initiated at the southernmost edge of the already existing southern Californian asthenospheric
window. During the Pliocene and the Quaternary, the thermal anomaly related with the asthenospheric window
and the crustal thinning linked with the opening of the Gulf of California led to MORB formation at spreading
centres of the Gulf and to partial melting of slab remnants to generate adakites, for example in Isla San Esteban.
This thermal event may have also triggered the partial melting of previously metasomatized supraslab mantle to
produce medium-K to high-K calc-alkaline magmas along the Main Gulf Escarpment and in the Gulf of California.
http:www.geologia-son.unam.mx/
V24A-05
Mafic Volcanism Along the Sinaloa Coast, Mexico, and its Relation to the Opening of the Gulf of California
We report on new localities with mafic volcanism along the Sinaloa coast, which record changes in the magma generation processes along the eastern margin of the Gulf of California. South of Culiacán, Sinaloa, isolated outcrops of basaltic lavas built a ca. 60 km long belt aligned to the SE. The similarity in the mineralogy and composition of the lavas suggest that these outcrops could have been part of a single flow. Lavas contain abundant plagioclase (up to 3 mm), and olivine (up to 1.5 mm) phenocrysts, and scarce clinopyroxene, in a relatively coarse matrix. In multiement diagrams, the lavas show the negative Nb and Ta, and positive Pb and Sr anomalies characteristic of subduction related rocks. The age determination of these rocks is in process, nevertheless, rocks with similar compositions are known from ~11 Ma mafic dikes that outcrop in southern Sinaloa. The Pericos volcanic field, located about 25 km to the NW of Culiacán is composed by lava flows, shield volcanoes, and cinder cones of basaltic composition that cover an area of aprox. 20 x 32 km, and have a well preserved morphology suggestive of a Pliocene-Quaternary age. Lavas are porphyritic and contain olivine, plagioclase and clinopyroxene in a microcrystalline matrix. Some lava flows contain abundant megacrysts of green clinopyroxene (up to 8 cm), olivine (up to 1 cm), and/or plagioclase (up to 1 cm), or aggregates of olivine and clinopyroxene. Trace element abundances are remarkably uniform among all analyzed samples and are characteristic of intraplate magmas. Rocks with very similar composition, mineralogy, and also containing megacrysts, have been reported in the Pliocene Punta Piaxtla and Mesa Cacaxtla, located 200 km to the SSE at the Sinaloa coast. Those similarities indicate that mafic intraplate volcanism related to the opening of the Gulf of California is more broadly represented in the area than previously considered.
V24A-06
A possible connection between post-subduction arc magmatism and adakite-NEB rock association in Baja California, Mexico
Late Miocene to Recent arc-related magmatism occurs in Baja California, Mexico despite the cessation of plate subduction along its western margin at ~12.5 Ma. It includes calcalkaline and K-rich andesites, tholeiitic basalts and basaltic andesites, alkalic basalts similar to many ocean island basalts (OIB), magnesian and basaltic andesites with adakitic affinity (bajaiites), adakites, and Nb-enriched basalts (NEB). A popular model for the close spatial and temporal association of adakite (plus bajaiite) and NEB in Baja California is these are due to melting of the subducted Farallon/Cocos plate, which in turn is caused by the influx of hot asthenospheric mantle through a window created in the subducted slab directly beneath the Baja California peninsula [e.g., Benoit, M. et. al. (2002) J. Geol. 110, 627-648; Calmus, T. et al. (2003) Lithos 66, 77-105]. Here I propose an alternative model for the cause of post-subduction magmatism in Baja California in particular and origin of adakite-NEB rock association in general. The complicated tectonic configuration of the subducting Farallon/Cocos plate and westward motion of the North American continent caused western Mexico to override the hot, upwelling Pacific mantle that was decoupled from the spreading centers abandoned west of Baja California. The upwelling asthenosphere is best manifested east of the peninsula, beneath the Gulf of California, and is most probably due to a tear or window in the subducted slab there. The upwelling asthenosphere is compositionally heterogeneous and sends materials westward into the mantle wedge beneath the peninsula. These materials provide sources for post-subduction tholeiitic and alkalic magmas. Portions of tholeiitic magmas directly erupted at the surface produce tholeiitic lavas, but some get ponded beneath the crust. Re-melting and/or high-pressure fractional crystallization of the ponded tholeiitic magmas generate adakitic rocks. Alkalic magmas directly erupted at the surface produce OIB-like lavas but those that get contaminated during transit produce NEB. The influx of asthenosphere also provides thermal energy to melt the upper portion of the mantle wedge - producing calc- alkaline lavas, and the amphibolitized deeper portion of the wedge - producing bajaiites, after the cessation of subduction in Baja California.
V24A-07
QUATERNARY ADAKITE - NB-ENRICHED BASALT ASSOCIATION IN THE WESTERN TRANS- MEXICAN VOLCANIC BELT: IS THERE ANY SLAB MELT EVIDENCE?
A spatial and temporal association between adakitic rocks and Nb-enriched basalts (NEB) has been recognised for the first time in the western sector of the Trans-Mexican Volcanic Belt in the San Pedro-Cerro Grande Volcanic Complex (SCVC). The SCVC is composed of subalkalic intermediate to felsic rocks, spanning in composition from high-silica andesites to rhyolites, and by the young transitional hawaiite and mugearite lavas of Amado Nervo shield volcano. Intermediate to felsic rocks of the SCVC show many geochemical characteristics of typical adakites, such as high Sr/Y ratios (up to 180) and low Y (< 18 ppm) and Yb contents. Mafic Amado Nervo rocks have high TiO2 (1.5-2.3 wt), Nb (14-27 ppm), Nb/La (0.5-0.9) and high absolute abundances of HFSE similar to those shown by NEB. However, the Sr and Nd isotopic signature of SCVC rocks is different from that shown by typical adakites and NEB. Although the adakites-NEB association has been traditionally considered as a strong evidence of slab-melting, we suggest that other processes can lead to its generation. Here we show that parental magmas of adakitic rocks of the SCVC derive their adakitic characteristic from high pressure crystal fractionation processes of garnet, amphibole and pyroxene of a normal arc basalt. On the other hand, Amado Nervo Na- alkaline parental magmas have been generated by sediment melting plus MORB-fluid flux melting of a heterogeneous mantle wedge, consisting of a mixture of depleted and an enriched mantle sources (90DM+10EM). We can not exclude a contribution to the subduction component of slab melts, because the component signature is dominated by sediment melt, but we argue that caution is needed in interpreting the adakites-NEB association in a genetic sense